This invention relates to an optical circulator which is a kind of optical non-reciprocal device for use as an optical functional device, for example, in an optical communication system, and which is arranged to output an incident light from a first input/output port to a second one, an incident light from the second input/output port to a third one, an incident light from the third input/output port to a fourth one and an incident light from the fourth input/output port to the first one.
A conventional optical circulator is composed of first and second polarizing prisms rotated by 45.degree. relative to each other about their optical axis and a Faraday rotator disposed between the first and second polarizing prisms. The Faraday rotator is an element so constructed that the direction of polarization of an incident light thereto is rotated by 45.degree. by external magnetization. The polarizing prism has the function of changing the optical path of an incident light thereto in accordance with its direction of polarization. The conventional circulator performs its function through utilization of the abovesaid function of the polarizing prism and the relationship between the rotation of a polarized light by the Faraday rotator and the positions of the first and second polarizing prisms rotated by 45.degree. relative to each other about their optical axis. In another conventional optical circulator, use is made of four Faraday rotators wherein the polarized light rotating angle is 45.degree., and these Faraday rotators are disposed so that optical paths respectively passing therethrough intersect one another at one position and one polarizing prism is disposed at the point of intersection. This optical circulator also utilizes the rotation of light polarization by the Faraday rotators and the aforesaid function of the polarizing prism.
These conventional optical circulators respond only to a single linearly polarized light predetermined for each input port and incident light components other than the linearly polarized component become optical loss or leakage. In an optical fiber which is indispensable as a transmission line for optical communications, however, it is known that the polarized state of light is not constant. That is, even if an incident light is a linearly polarized light at an input port, the linearity of the polarized light is lost by propagation over as short a distance as, for example, about 20 cm. Consequently, although light of a semiconductor laser widely employed as a light source for optical fiber communication is linearly polarized, when the semiconductor laser is coupled with an optical fiber, the output light from the optical fiber does not maintain the linearity of the polarized light. Accordingly, an optical circulator intended to be coupled with the optical fiber must be adapted to respond to lights polarized in any directions so as to reduce the loss.
An object of this invention is to provide an optical circulator which is free from polarization dependency, that is, capable of responding to lights polarized in any arbitrary directions.
Another object of this invention is to provide an optical circulator which has excellent isolation characteristics.
Still another object of this invention is to provide an optical circulator whose parts are joined together to be formed as a unitary structure mechanically.